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The webmaster, Pat Cryer, as a young child

Mercury barometers for
forecasting the weather

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Although I grew up used to hearing weather forecasts on the radio (or the wireless as it was then called), older people around me still used the methods that they knew from earlier in the century before home radios became common. One of these was the barometer.

Barometers measure air pressure. How air pressure forecasts weather is explained on a separate page.

Two types of barometer were used in the home - the mercury barometer which is the subject of this page and a later more common type called the aneroid barometer.

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How the mercury barometer works

It is probably easiest to explain how the mercury barometer works, by asking you to think back to something that you must have done at some time in your life. Surely you must have put an ordinary drinking glass under water, tipped it up and lifted it partially out of the water. The glass stays full of water. This doesn't fall out until the glass is lifted completely out of the water. Even then, a slight resistance can be felt.

Glass of water upside down in a bowl of water, supported by air pressure

Glass of water upside down in a bowl of water, with the water in the glass supported by air pressure

The reason for this is that the pressure of the air on the surface of the water is keeping the water in the glass.

In fact only if the glass were made about 10 metres tall, would the weight of the water inside become too great for the force of the air pressure. Then the level of the water would drop, leaving a vacuum above it.

Because air pressure changes with the weather, so the height of the water column would change, and this change could be used to measure air pressure. However a receptacle of water this tall is impractical. A shorter and narrower one is necessary which means a closed tube of a much heavier liquid. The answer is a tube of mercury.

Mercury is expensive and contact with it is bad for people. So the glass containing it is made as a narrow tube and the bowl is made small and covered apart from a small hole which is open to the air. The arrangement does still require the closed tube to be tall, but at less than a metre, it is practical. Being bulky, though, this mercury barometer was generally used as a display item fixed to an indoor wall and usually housed in a rather elegant polished wood case. A scale behind the top of the tube allowed variations in the height of the mercury column to be measured. (Standard atmospheric pressure is taken to be 760mm of mercury, but it does of course vary from place to place and with the weather.)

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Mercury barometers in homes

mercury barometer

A mercury barometer, photographed in Milton Keynes Museum. Its old-style affluence is clear from its decoration and inscriptions on its face.

detail of a mercury barometer

Detail of a mercury barometer, showing the small mercury reservoir, open to the air through a small space and the bulb associated thermometer for measuring temperature.

Mercury barometers were popular in Victorian times, but as mercury was expensive, they usually only found their way into somewhat affluent homes. I did see a few around, though, when I was growing up in the 1940s.

Mercury barometers are more difficult to read than the later and more common aneroid barometers, but the term, "The mercury is falling (or rising)" only relatively recently went out of common speech to describe changes in the weather.

If you can add anything to this page, I would be pleased to hear from you.

Pat Cryer, webmaster

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